Yes, but it depends a bit on how you want to approach this.

1. The easiest way is to make sure Firefly III imports them as transfers in the first place. This depends on the data quality of Mint, which I don't know a lot about.
2. You can also use rules to select transactions with certain properties and automatically convert them to transfers.
3. Select each transaction individually and select "convert to transfer" from the dropdown.

Also perhaps helpful: https://github.com/orgs/firefly-iii/discussions/7983

I'll add my experience since I'm also trying to import from Mint to Firefly.

I tried for several days/weeks to use the Firefly importer to import from the Mint transactions.csv, however I could never get it to work right. Mint stores a transfer as a credit/debit, but over several banks and several years, the descriptions were not enough to really resolve what accounts are part of the transfer. Trying to remap the data to turn descriptions into Opposing Account Names would just lead the importer to completely time out and or crash due to the data sizes involved or would not be useful in the case of "Transfer from Checking"--Which checking? And when it did import, I ended up with duplicates (when the importer would create a transfer on both sides.)

Ultimately, that has lead me to instead iterate on this using a custom Python + Pandas script that would handle the import by identifying likely transfer pairs based on dates, amounts, and descriptions. It's not yet working, nor is it at all clean, but it's so far avoided the above problems. I intend to document and share it once it's working better.